fix: view-queue link + agent inbox shows unread messages only

fix(dashboard): update 'view queue' link to /builds.html

The build queue moved to its own /builds.html page. The queue-summary
'view queue →' link in tabs.js still pointed at /core.html, so clicking
it landed on the wrong page.

fix(broker): filter agent inbox to unread (acked_at IS NULL)

recent_for was returning all messages regardless of ack state, so the
agent inbox showed everything even after 'mark all read'. Now filters
to acked_at IS NULL — mirroring exactly what mark_all_read drains —
so the inbox empties on reload after the operator drains it.

frontend/packages/dashboard/src/tabs.js

@@ -826,7 +826,7 @@ window.marked = marked;
         el('span', { class: 'glyph spinner' }, '◐'), ' ',
         el('strong', {}, 'build queue'), ' — ',
         parts.join(' · '), ' ',
-        el('a', { class: 'queue-summary-link', href: '/core.html' }, 'view queue →'),
+        el('a', { class: 'queue-summary-link', href: '/builds.html' }, 'view queue →'),
       ));
     }